Disc drives are digital data storage devices which utilize one or more rotatable, magnetic discs to store and retrieve computer data. A plurality of controllably positionable read/write heads are used to selectively magnetize tracks on the disc surfaces to store the data, and to transduce the selective magnetization of the tracks to retrieve the data to a host computer in which the disc drive is mounted.
Typically, each track includes a number of servo fields which are periodically interspersed with user data fields. The user data fields are used to store computer data and the servo fields store prerecorded servo information used by a disc drive servo system to control the position of the heads.
The servo system operates in two primary modes: seeking and track following. During a seek, a selected head is moved from an initial track to a destination track on the corresponding disc surface. Generally, a velocity profile defines a desired velocity trajectory for the head as the head is first accelerated and then decelerated to move from the initial track to the destination track. As the head nears the destination track, a settling mode is entered whereby the servo system attempts to settle the head onto the destination track in a minimum amount of time. Thereafter, the servo system switches to the track following mode of operation so that the head is maintained over the destination track until a subsequent head switch or seek operation is performed.
Each head includes a write element to write the data to the tracks and a read element which reads the user data and the servo data from the track. A typical write element comprises a thin film inductive coil having a write gap that, when subjected to a time-varying write current indicative of the data to be stored, generates a corresponding time-varying magnetic field across the gap which selectively magnetizes the tracks. A typical read element construction includes a magneto-resistive (MR) material characterized as having a changed electrical resistance when subjected to a magnetic field of selected orientation. Stored data are recovered by passing a read bias current through the read element and detecting changes in voltage thereacross in response to the magnetization of the tracks. Although head constructions can vary, the effective size of the write element is typically larger (with respect to track width) than the size of the read element, and the effective centers of the read and write elements may be physically offset within the head.
To maintain data integrity and high data transfer rates, it is critical that the read and write elements be respectively maintained as close as practicable over the center of each track during read and write operations. For example, even if data are properly written in a centered relationship over a selected track, attempting to subsequently read the data while the head is positioned a sufficient distance away from the center of the track may result in an unacceptable number of read errors, due to the inability of the read element to properly detect the written data, as well as the potential interference from the selective magnetization of an adjacent track. More significantly, writing data too far away from the track center can prevent subsequent recovery when the head is centered over the track, and can also corrupt data stored on the adjacent track.
Thus, disc drives typically utilize read fault and write fault thresholds to minimize the occurrence of read errors and data overwriting. These thresholds are usually expressed as a percentage of track width and define zones about the center of the tracks in which safe reading and writing can take place. For example, a typical read fault threshold might be established at .+-.10% of the track width, so that read operations are enabled only while the head is less than 10% of the track width away from the center of the track. Similarly, a typical write fault threshold might be established at .+-.17%, so that write operations are enabled only while the head is less than 17% away from the center of the track. During read and write operations, the servo system continually monitors the position of the respective elements and causes the interruption of the respective operation if the threshold is reached or exceeded. The thresholds are determined during disc drive design and are intended to balance various factors including track density, acceptable read error rates, expected variations in the sizes of the read and write elements, and acceptable data transfer rates.
It will be recognized that extended read and write operations often involve the accessing of multiple tracks by a single head, and can further involve the accessing of multiple tracks by multiple heads. In order to maximize data transfer performance of a disc drive, it is desirable to begin reading or writing data as soon as the head is sufficiently settled onto each accessed track. In practice, disc drives typically monitor the position of the head as it is settled onto each track and initiate the respective read or write operation as soon as the head is within the respective fault threshold (and the head is over the desired user data field).
Hence, while tightening the read and write fault tolerances of a disc drive would likely result in corresponding improvements in error rate performance by the drive, it would also undesirably degrade the transfer rate performance of the drive, as the drive would have to wait longer to ensure the head is sufficiently settled onto the destination track (and maintained in proper relation thereto) before commencing the respective read or write operation.
Such tightening of the read and write fault thresholds would also place greater strains upon the servo system in maintaining the heads within the defined acceptable read and write zones, likely resulting in a greater number of interruptions in the data transfer process as read and write faults are declared and resolved. Tightening the read and write fault thresholds would also generally result in a reduction of the operational shock performance characteristics of the drive, as the drive would be less tolerant to the application of external shocks and vibrations that tend to move the heads away from the centers of the followed tracks.
Consumer demand for disc drives with ever increasing data storage capacities and transfer rate performance levels has led disc drive manufacturers to attempt to achieve greater data storage densities and read/write channel capabilities in successive generations of drives, including balancing the sometimes conflicting requirements of enhanced error rate and transfer rate performance. Accordingly, it is to the furtherance of these efforts to improve disc drive performance that the present invention is directed.